Traditionally, there have been investigations of color recording methods for color hard copies based on ink jet, electrophotography and thermal transfer recording, for instance.
Among these methods, thermal transfer recording has some advantages, including easy operation and maintenance and permission of equipment size reduction and cost reduction. There are two modes of this thermal transfer recording method.
In one method, a transfer sheet having a melting ink layer on the support is imagewise heated by a laser beam or thermal head to melt-transfer the melting ink layer onto an image-receiving sheet for thermal transfer recording. In the other method, the heat diffusion transfer method, an ink sheet for thermal transfer recording having on the support an ink layer containing a heat diffusible dye (sublimation dye) is used to diffuse and transfer the heat diffusible dye onto an image-receiving sheet for thermal transfer recording.
The heat diffusion transfer method has recently drawn much attention as yielding color images with continuously changing color density by superpose recording of cyan, magenta and yellow colors, since it permits image gradation control by changing the amount of dyes transferred according to thermal energy change in the thermal head.
However, the conventional heat diffusion transfer method has a drawback of essentially poor image preservability. In comparison with the recording images obtained by ordinary silver salt photographic methods, the storage period is shorter, which hampers the practical application of this method to personal identifying photographs and other fields where high image preservability is essential.
The present invention has been made to solve the problems described above. Accordingly, the present invention relates to a thermal transfer image-recording material which offers improved image preservability, prevents the falsification of the image or the thermal transfer image-recording material itself and offers improved durability of the thermal transfer image-recording material, and a method of its production.
Developed with the aim of accomplishing this object, the present invention provides a thermal transfer image-recording material comprising an image-bearing image-receiving layer, a UV-absorbing resin layer and a UV-setting resin layer, all of which are formed on the support in this order, and a method of producing a thermal transfer image-recording material wherein the image-receiving layer of an image-receiving sheet for thermal transfer recording, composed of the support and the image-receiving layer, and the ink layer, containing a heat diffusible dye, of an ink sheet for thermal transfer recording are superposed and imagewise heated to transfer an image to the image-receiving layer, this image formation surface is covered with a UV-absorbing resin layer and then covered with a UV-setting resin layer, which layer is then set by UV irradiation.
Traditionally, a wide variety of ID cards have been used, including identification certificates, driving licenses and membership certificates. The ID card usually bears a personal figure image for identification of the card owner and various pieces of other information. The personal figure image can be prepared as a gradation-information-bearing image because it usually has varied density. The various pieces of other information include the address, name, date of birth and position of the card owner and the validation date of the card in the case of personal identification certificates, and the date of birth, name, license number of the card owner and the license category in the case of driving licenses. These pieces of information, written in characters, numerical figures, symbols, etc., can be prepared as a character-information-bearing image.
Currently there are two methods of forming a gradation-information-bearing image for ID cards, namely the sublimation transfer method and the silver halide photographic method. Irrespective of which method is used to form the gradation-information-bearing image, a key to ID cards is to perfectly prevent the forgery and alteration thereof. It is very evident why ID cards should not be forged or altered.
Traditionally, to prevent the forgery and alteration of ID cards, it has been recognized as effective to laminate the ID card with a transparent sheet (this method is also referred to as the laminate method for short) or to coat the ID card with a UV-setting resin and then irradiate ultraviolet rays to set the UV-setting resin to form a set film (this method is also referred to as the UV irradiation method for short).
However, even when the ID card having a gradation-information-bearing image formed on the image-receiving layer by sublimation thermal transfer is laminated with a transparent sheet, the image-receiving layer and the transparent sheet can easily be detached from each other, since the image-receiving layer itself is a thermoplastic resin sheet, for instance, and the transparent sheet is also a thermoplastic resin sheet. Therefore, transparent sheet lamination cannot perfectly prevent the forgery or alteration.
Also, the gradation-information-bearing image formed on the image-receiving layer by sublimation thermal transfer poses a problem of image damage by heating upon lamination of the transparent sheet on the ID card because this image is formed by a sublimation dye. In other words, in case of excessive heat being added, in order to obtain a sufficient adhesion, the gradation-information-bearing image formed on the image-receiving layer by sublimation thermal transfer has no endurance against the lamination treatment.
When laminating the ID card with a transparent sheet, the transparent sheet usually has an area larger than that of the ID card. Specifically, a transparent sheet having an area larger than that of the ID card is placed on the gradation-information-bearing image formation surface of the ID card, followed by heat treatment, after which the transparent sheet's portion sticking out from the ID card is cut out. Therefore, the ID card lamination method produces transparent sheet cutting dust; a problem of occurrence of much cutting dust is posed where a large number of ID cards are prepared.
When a protective layer is formed by UV irradiation on the surface of an ID card having a gradation-information-bearing image formed on the image-receiving layer by sublimation thermal transfer, coating a UV-setting resin on the gradation-information-bearing image surface results in blurs in the gradation-information-bearing image formed by sublimation dye, which poses a problem of loss of image distinctness. Since the ID card will not ensure the identification of the card owner, if the personal figure image, a gradation-information-bearing image, is blurred, this UV irradiation method is critically faulty so that it cannot be adopted for the preparation of ID cards.
On the other hand, with respect to the ID cards having a gradation-information-bearing image formed by the silver salt photographic method, there is no problem as with the ID cards having a gradation-information-bearing image formed by the sublimation thermal transfer method, it requires much time to prepare ID cards because it requires a large number of processes such as development, fixation and bleaching to form the gradation-information-bearing image. Therefore, the method of ID card preparation based on silver salt photography is not applicable at all where a large number of ID cards should be prepared rapidly.
The present invention has been developed in the circumstances described above. The object of the invention is to overcome the problems described above and provide a unforgeable, unalterable card-sized image-recording material having a distinct gradation-information-bearing image with high durability using no laminate films, and a method of rapidly producing such an excellent card-sized image-recording material wherein the image is not damaged by sublimation dye even in UV irradiation during the production process and no cutting dust is produced as with the use of laminate film.
With the aim of accomplishing the object described above, the present inventors made investigations focusing mainly on some points, which are described below with reference to an example of issuing ID cards such as driving license certificates.
The inventors first directed their attention to the fact that sublimation thermal transfer is very advantageous over silver salt photography in that it is much more rapid in smooth formation of a color photographic facial image. The inventors also directed their attention to the fact that operation and equipment can often be simplified with advantage by printing ID data and common data such as those in characters or codes by hot melt thermal transfer or sublimation thermal transfer.
In this case, however, the inventors considered that a satisfactory result can be obtained by printing the gradation-information-bearing image by sublimation thermal transfer while printing the image which should not necessarily be a gradation-information-bearing image by hot melt thermal transfer, and that these methods may be appropriately selected as the case may be.
In the case of images printed by sublimation thermal transfer (e.g., color photographic facial images), there is a problem of blurs and discoloration in the sublimation dye image due to heat upon lamination treatment when it is attempted to laminate the image in perfect adhesion with the laminate material to protect the image or the image recording material or to prevent the falsification of the image.
The inventors thus directed their attention from the lamination method to the traditional method in common use for silver salt photographic images, in which a protective film comprising a UV-setting resin film is formed on the image, to use it to provide protection and durability for the image or image-recording material. This protective film formation method can easily be achieved by coating a coating agent containing a UV-setting resin on the entire or desired partial surface of the image-recording material and setting the resin or monomer by UV irradiation, which method should ensure more rapid obtainment of the desired effects with no heating.
However, this method, in which a protective film comprising a UV-setting resin layer is formed, was found to pose other problems, such as sublimation dye blurs in the sublimation thermal transfer image by the resin, monomer or solvent during coating the coating agent, image damage during UV irradiation and inhibition of the setting of UV-setting resin by sublimation dye.